I have to agree with the others but I would say try out a 998 no rocker problems to worry about,
you have an iconic ducati in the 900 the next iconic Ducati would be a 748-998 bike. Very easy
to work on no (major problems) like the plastic tank bikes and everyone loves them even non riders.
Find one and try it on for size they are small bikes and more comfortable than any sport classic that is
not a GT.
I agree with almost all of this. But I'd still recommend a 916 or 996 along with the 998.
I kinda did the same as the OP but backwards. I got a 996, and it was everything I thought it would be, and a good deal more. It is easy to work on...which is a plus when you're maintaining several bikes in the garage. It is also a plus when you do all your own work.
The Desmoquattro engine is an evolution of the Desmodue when viewed from a purely mechanical perspective. The crankcase is similar, clutch parts are interchangeable as are the tools to work on the basic stuff. It's only slightly more difficult to work on than the air cooled Desmodue.
Riding performance wise, it's all that you describe your Supersport being, only more of it. More of it all over the spectrum. I'm not saying better....just more of everything.
Ergonomically, there's no comparison. The Supersport is much more comfortable for a long day in the saddle. That stated up front, the 996 isn't horrible. I'd say it's no more uncomfortable than my 1st gen GSXR.
It wouldn't be my first recommend for a 2 or 3 hour slog up the slab but once on the back roads, it's a joy to ride. If you spend more time on one or the other, you'd have the perfect tools for both by adding an iconic superbike to the garage.
For a people to bike sizing reference, I too am 5'9" with a short inseam to boot.....sean